This invention relates to a process for producing sinters. More particularly, it relates to a process which comprises mixing a metal or ceramic powder with a binder that contains a thermoplastic resin and wax as main components, injection molding the mixture, dewaxing the molded article (removal of binder) and sintering the dewaxed article, whereby sinters of a highly precise and three-dimensionally complex shape can be produced in large quantities.
It is known to produce metal or ceramic sinters by a process that comprises mixing the starting powder with a thermoplastic binder, shaping the mixture, dewaxing the shaped article (removing the binder) and sintering the dewaxed article. If sinters having a complex three-dimensional shape need to be produced with high dimensional precision, the shaped article is conventionally dewaxed as it is buried in inert powders such as those of ceramics (e.g. alumina) so as to prevent the shaped article from deforming or cracking during the dewaxing (thermal dewaxing). However, this dewaxing method is not practical for commercial applications since it is very difficult to remove ceramic and other inert powders from the surface because of the fragility of the dewaxed article.
Therefore, a first object of the present invention is to provide a binder that can be removed from a shaped article of the starting powder and said binder without causing deformation or cracking even if the article is not buried in an inert powder, as well as a process for producing a sinter using that binder.
The atmosphere for sintering shaped metal powders is generally selected from among reducing atmospheres (e.g. hydrogen gas and dissociated ammonia gas) or inert atmospheres (e.g. argon, helium and vacuo) depending upon the type of metal powder to be used. Particularly in the case of metals or alloys such as stainless steel that are subject to deterioration in performance such as corrosion resistance or mechanical properties on account of carbon pickup (carburization) from molding aids (e.g. binders and lubricants), it is conventional to perform sintering under vacuum so as to remove carbon by making use of the following reaction:
C+MO (metal oxide in the metal powder).fwdarw.M+CO.
However, the sintering under vacuum involves metal evaporation from the shaped article at elevated temperature and this makes it very difficult to achieve close control on the dimensional precision of the final product. Since the amount of metal evaporation depends on both temperature and the degree of vacuum, nonuniformity in the temperature in the sintering furnace or the inter-batch differences in the furnace temperature and the degree of vacuum will cause variations in the dimensions of the sinter. Further, the ultimate pressure in the vacuum sintering furnace will depend on the number of samples to be charged and is usually difficult to control in a precise manner.
Therefore, a second object of the present invention is to provide a sintering method by which sinters of high dimensional precision can be manufactured from metals and alloys such as stainless steel that inherently are subject to deterioration in performance such as corrosion resistance or mechanical properties on account of carbon pickup from molding aids (e.g. binders and lubricants).
When metal or ceramic sinters are produced from injection molded articles by a process including dewaxing and sintering steps, it is conventional to perform the dewaxing step in an air or an inert gas atmosphere under atmospheric or pressurized condition but this step has not been practiced in vacuo in order to prevent blistering.
When dewaxing the shaped article in an air or an inert gas atmosphere under atmospheric or pressurized condition, the decomposed gas of the binder will stay around the injection molded article to react with the binder component remaining in the latter, thereby causing unwanted deformation. Under the circumstances, a plurality of shaped articles must be charged into the dewaxing furnace with the distance between adjacent articles being kept sufficiently large to avoid the possible deformation of the articles. However, this has limited the amount of shaped articles that can be charged into the furnace. In addition, deformation, cracking or blistering will occur at rapid heating rate, so the temperature in the furnace must be raised at a slow rate of 1.degree.-5.degree. C./h but this has prolonged the time necessary to complete the dewaxing of the shaped article.
With a view to achieving dewaxing at a faster speed, Unexamined Published Japanese Patent Application No. 28303/1989 proposed that an injection molded article comprising a magnetic alloy powder and a thermoplastic resin be dewaxed in a vacuum atmosphere. However, the vacuum dewaxing method disclosed in that patent involves temperature elevation at such a high rate of 2.degree.-10.degree. C./min that if it is applied to the purpose of dewaxing an injection molded article comprising a metal or ceramic powder and a conventional binder, blistering, deformation or cracking will occur, making it impossible to accomplish mass-production of sinters having high dimensional precision.
Therefore, a third object of the present invention is to provide a process for producing sinters that is free from the aforementioned problems of the prior art.